This invention relates to a method of introducing a liquid into a hole having a small diameter, as well as to a liquid introducing apparatus which uses this method. More particularly, the invention relates to a technique for reliably introducing a liquid into a microhole formed in a substrate. By way of example, the invention can be applied effectively to the introduction of a liquid for a pretreatment when applying a plating treatment to the inner wall surface of a microhole formed in an electrically conductive film for the purpose of communicating both sides of a printed circuit board.
Ordinarily, when subjecting a substrate having a microhole to a treatment such as washing, plating or etching by a wet method using an aqueous solution, it is necessary to bring the liquid into contact with the entire surface of the substrate. However, though the liquid makes good contact with the outer surface of the substrate, it is difficult for the liquid to penetrate the interior of the microhole and to contact its inner wall surface.
FIGS. 8A and 8B are schematic sectional views for describing the conventional method of introducing liquid. When a substrate 1 having a microhole 2 is immersed in a liquid 5, as shown in FIG. 8B, after having been left standing in a gas 7 such as air (represented by the dots), as illustrated in FIG. 8A, the gas 7 remains in the microhole 2 in the form of a bubble W. As a result, portions 2a of the inner wall surface of the microhole are not contacted by the liquid 5.
For example, when a plating film is formed on the inner wall surface of the microhole 2 in the manufacture of a printed circuit board, a large number of steps using a suitable aqueous solution are carried out, such as a degreasing treatment, a catalytic treatment, an electroless plating treatment, an electroplating treatment and a water-washing treatment performed between each of these treatment steps. Consequently, when the air bubble W is left in the microhole 2, a plating film cannot be formed here and this can cause a major malfunction.
Recent printed circuit boards used in portable video cameras, personal computers and word processors are becoming increasingly smaller in size and the wiring patterns on them are becoming more dense. As a consequence, microhole diameter is gradually being reduced and a diameter on the order of 0.1 mm is now required whereas a hole diameter of 0.3 mm sufficed in the past.
A method of introducing liquid into such microholes has been proposed in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-154797 and 62-190794, by way of example.
The art proposed in Japanese Patent Application Laid-Open No. 62-154797 is a method of introducing a liquid following the expulsion of air from the interior of a hole by vibrating a substrate using a vibratory degassing apparatus. Though this method is capable of readily removing air from the proximity of the openings at both ends of the hole, difficulty is encountered when it is attempted to expel the air from the central portion of the hole interior and introduce the liquid to this portion, and hence the method is beset by problems in terms of reliability.
The art proposed in Japanese Patent Application Laid-Open No. 62-190794 is a method of introducing a flux solution into a hole following elimination of air from the hole by reducing pressure in a vacuum degassing tank. This method would be highly reliable if it were capable of establishing a perfect vacuum. However, since it is required that the entire tank be hermetically sealed, the entire apparatus must have a structure which is highly rigid and which will not allow leakage of air. A problem that arises is high cost for the equipment. In a case where a gas containing moisture is drawn in using an ordinary vacuum pump, oil recirculating within the pump is broken down owing to the effects of moisture, and therefore this arrangement is not suited for use in a treatment that relies upon a liquid. In order to avoid this, common practice is to effect a reduction in pressure using a water jet pump. However, since a water jet pump exhibits but a small decompressing capability, obtaining the desired vacuum requires an extended period of time and productivity is low.
U.S. Pat. No. 4,781,943 discloses a process for pretreatment before plating through-holes of printed circuit boards. According to the process disclosed, a liquid is introduced into a hole by immersing a circuit board in vapor for about one minute, extracting it from the vapor and then immersing it, less than about 15 seconds after extraction, in running water for about one minute, the temperature of the water being less than 25.degree. C.
However, when the circuit board is dipped into the vapor and then lifted out, the vapor is no longer capable of remaining inside the hole and the liquid cannot be introduced owing to the effect of volumetric contraction caused by a drop in the temperature of the vapor, which is a gaseous state.
Accordingly, the aforementioned U.S. Pat. No. 4,781,943 further discloses introducing a treating solution into a hole by boiling water or a treating solution that is soluble in water, immersing the substrate and then cooling the treating solution without raising the substrate out of the boiling solution.
In accordance with this proposal, however, the treating solution is boiled, the substrate is immersed and then, without extracting the substrate, the treating solution, which is in the boiling state, is cooled. The problem that arises here is low treating efficiency because the boiling and cooling cycles take time.